Using radiolabeled free fatty acids, a mathematical model was developed to calculate their transfer constants (K*FAA) and rates of incorporation from plasma (JFFA) of plasma FFA into individual brain regions; [9, 10 -3H]14 palmitate (PA) is incorporated into the sn-1 position os phosphatidylcholine; [1-14C] arachidonate (AA) is incorporated into the sn- 2 position of phosphatidylinositol and phosphatidylcholine; [1-14C] decosahexaenoate (DHA) is incorporated into the sn-2 position of phosphatidylethanolamine. Incorporation of 3H PA into intracerebrally implanted Walker 256 carcinosarcome was 3-6 fold higher than in control brain in rats. Incorporation of 18 F-dl-erythro-9, 10-difluoropalmitate into brain phospholipids was similar to that of 14 C-PA. Therefore, this tracer may be a useful prove for studying brain lipid metabolism in humans with positron emission tomography. Administration of neurotransmitter agonists to rats resulted in increased brain incorporation of 14 C-AA (serotonin, cholinergic) and 14 C-DHA (cholinergic), but not 3 H-PA. Increases were greater in areas containing receptors linked to lipid turnover.